Hello all,

I am not sure if this is the right place to ask this question, so my apologies if this should be posted somewhere else on the forum.

I found the plans for Custom Rocket's Landviper over on the rocket plans side of the site. http://plans.rocketshoppe.com/custo...01/cus10001.htm.

The Landviper is a 3 motor cluster, housed inside a BT-60. It has 6 trapezoidal fins, all located at the aft end of the rocket.

Before I go much further, I would like to say that I am a rather novice rocketeer with a few packaged rockets built and a scratch built Estes Blackhawk made from cardstock under my belt. However, I think this would be a fun rocket to try an experiment to induce spin with fin cant. Now that I think about it, a 3 motor rocket may not be the ideal choice for an experiment, as it'll run about 10 bucks a launch for motors alone. I know spin can be done with added fin cant tabs, but I want to build it into the rocket design.

I started playing around with the trial Rocksim program to get an idea about the max height at apogee (since that is the limit of the launch simulation in the trial version). But, Rocksim wont allow me to put the 3 motor cluster inside a BT-60. It will allow it to fit inside a BT-70, which I did and put in a transition from BT-70 to the nose cone which I left with the original ellipse BNC-60MS on Uncle Mike's. Rocksim put the apogee at almost 1500 feet with C series motors. I did not see if there was any way of adding fin cant to the design in Rocksim.

So, with that, I have a couple of questions regarding the fins and the angle needed to impart spin.

1st off, do I need all 6 fins? I've read (Apogee Newsletters, Tim Van Milligan) that more than 4 is overkill, and is purely an aesthetic choice. Personally, I want to add all 6 fins for the aesthetics reasons alone, however, I understand that fin cant will lower the overall altitude. I guess my question is, would there be a significant decrease in drag by limiting the rocket to 3 or 4 fins? If there is a significant reduction in drag it may offset the reduction in overall altitude due to the fin cant, thereby keeping the apogee above the 1000 foot mark. I will also round or asymmetrically airfoil the leading edges of the fins to reduce drag.

2ndly, I am operating under the impression that fin cant angle is the angle at which the root edge of the fin is attached to the body tube. So a square root edge fin would have a 0 degree cant, and stick up perpendicularly from the center of the body tube.

Finally, assuming I have my definitions correct, what would be a good choice for fin cant angle? Is a small angle, like 5-10 degrees enough to impart the spin? Or do I need a significant amount like 30 to 45 degrees to do it?

Any and all help, suggestions, critiques is greatly appreciated.

Basically ANY fin cant WRT the centerline/thrustline of the rocket will induce spin... even as little as 1-2 degrees is sufficient. OF course the amount of spin is directly proportional to the degree of cant, rotational inertia of the rocket (how long it takes to "spin it up"), and the amount of lift force generated by the fins themselves (combination of fin size and airspeed).

A cant of 45 degrees is HUGE overkill and will generate enormous "plate drag".

Multiple fins beyond four is more for aesthetics rather than anything else. Additional fins result in additional drag as you mentioned due to friction drag of air across the surface of the additional fins, plus interference drag caused by the additional fins sticking out in the airstream, and induced drag caused by airflow spiraling over the tip of the fin at non-zero angle of attack situations. I don't have data to back it up, but removing the additional fins and going with say 3, is going to reduce the drag MUCH less than the additional drag caused by canting the fins WRT the centerline of the rocket and the airstream its flying through.

In addition to producing rotation of the rocket by fin canting WRT the long axis of the rocket, roll can also be induced by the fins not being parallel to the surface of the tube (at an angle to the tube other than 90 degree straight out from it, and especially WRT other fins on the opposite side of the rocket) and also roll can be induced by the fins having a non-symmetrical airfoil sanded into them (like an airplane wing, flat on one side and curved on the other-- this generates lift forces perpendicular to the surface of the fin toward the curved side, and thus will induce a rolling motion since the force is generated off-center of the vehicle centerline, ESPECIALLY if it's coupled with an identical airfoil on the other side of the rocket (or all the fins are sanded in a similar wing airfoil all pointing in the same direction).

Hope this helps! OL JR

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As far as the angle to cant the fins, a very small angle is all that is needed. For example, I wanted to induce a spin on my Deuce's Wild to try and get a helix shaped smoke trail.

I drew the fin lines straight and put the bottom of the fin on one side of the line and the top on the other side of the line. That's all it needed to give it a good spin.

Needless to say, all of the fins should be canted in the same direction.

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Bob Harrington
NAR #62740 L1
AMA #46042
CMASS & RIMRA Member

Thank you guys for your responses. They have been extremely helpful, as my impression of fin cant was incorrect. I was thinking of it in terms of how the whole fin would appear to lean to one side if viewed from the nose cone down the length of the body tube.

But when Luke said that a 45 degree cant would induce major plate drag, it clicked. The cant is the degree in which the fins are parallel to the centerline or thrustline. Which makes a lot of sense. In fact, that was my first thought of how to induce spin. I thought that leaning the fins over would also produce the effect as well as looking cool. I imagine that it will create the roll/spin effect that you mentioned though with less of an effect.

Bob, thanks for your fin line suggestion. I think that will work for my project. I was hoping that you would happen upon this, as I saw your Deuce's Wild with fin cant project post. How much spin did that offset create? Did you have to modify the root edge to get a good solid bond between the fin and body tube?

Thanks again gentlemen!

Without testing it, I like the asymmetrical airfoil idea the best. Leave on edge of the fin perfectly straight, then sand the other edge into a proper airfoil like an airplane wing, sounded near the front edge, and tapering back towards the fins center line at the trailing edge. I think I will do this on my next build just for kicks.

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I agree with the rest, it takes just a few degrees of fin cant to make a rocket spin.

When I was testing an early prototype of the Odd'l Rockets Pipeline model I added too much angle to the tube fins.
It spun too fast and pretty much pulled itself apart!

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That slight amount of offset produces a moderate roll. Quicker than I would have thought but for what I was trying to accomplish, it was just about perfect.

The amount of offset was so slight that the root edge bonded just fine.

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Bob Harrington
NAR #62740 L1
AMA #46042
CMASS & RIMRA Member

I want to answer your questions on spin stabilization, but I cant.

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I love sanding.

I did one years ago and I just warped the tips of the fins.

I soaked the fin tips in water then wrapped just the tips around a Testor's paint stray can.

The results were a really cool spin on lift off, very fast, and with no streamer or parachute just a nose blow it had a helicopter like recovery.

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On my 6x upscale mega hi flyer the fins are ttw . With the motor mount in I set the fins with a angle finder at deg from 90 to the right . Due to the fins being so big the root was 27" 9.5 " tall to cancel out spin . It worked great

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